With the industrial development, industrial water and domestic water have been used more and more. The amount of discharge of various kinds of sewage and wastewater caused by such use has also increased.
Industrial water and domestic water should be treated into clean water and then discharged to rivers. However, this is not in line with the reality. As a result, pollution of rivers or lakes is increasing.
Various organic pollutants flowing into the rivers or lakes cause an abnormal multiplication of various microorganisms including algae. For this reason, a shortage of dissolved oxygen occurs, and the self-purification power of water is lost. As a result, the water becomes foul.
For this reason, various efforts are being made to improve water quality using a water circulating method. However, due to structural limits or power supply limits, commercialization has not yet proceeded favorably. Thus, a countermeasure against this is required.